The present invention relates to a process for the production of a three-dimensionally shaped sandwich structure comprising the steps of providing at least a core layer for the sandwich structure which comprises a compressible core material; positioning said core layer in a compression mould between a first and a second mould surface thereof, the first mould surface being formed by a first mould section and the second mould surface by a second mould section, the first and second mould sections being movable with respect to one another between a moulding and a demoulding position; and bringing the mould sections into their moulding position to mould said core layer into a three-dimensional shape between the first and the second mould surfaces; the core layer having two opposite main faces which are covered before and/or after the moulding step with at least one layer of a covering material to achieve said sandwich structure.
The compressible core layer is intended to keep the covering material during the moulding process near the surface of the sandwich structure so that a lightweight sandwich structure with good mechanical properties can be obtained. Due to their relatively low weight, sandwich structures are especially advantageous for use in vehicles such as cars, ships, planes. They can in particular be used as substrate layer for wall coverings such as door or roof trim parts. In practice, the compressible core layer is usually made of a non-elastic or non-stretchable material which lacks ductility such as corrugated paperboard or a honeycomb material. An important drawback of the presence of such non-elastic or non-stretchable core materials in the sandwich structures is that they are difficult to be moulded into complex, three-dimensional shapes. This problem has already been treated in a number of patent publications.
U.S. Pat. No. 4,119,451 discloses for example a method for manufacturing an automobile roof trim board comprising a corrugated paperboard as substrate and a skin or facing layer fixed thereto. The problem addressed in this patent is that during the press-forming of the corrugated paperboard cracks will occur therein at the location of the deep-drawn areas. These cracks reduce the strength of the trim board and may even be seen through the covering layer which is applied onto the moulded paperboard. To prevent or reduce the formation of such cracks, U.S. Pat. No. 4,119,451 teaches to make, prior to the press-forming process, notches in the corrugated paperboard at the location of the deep-drawn areas. However, it is clear that this solution can only be used in certain areas, more particularly in areas wherein holes will be cut out later for example to receive a room lamp. Moreover, it is not applicable to complexly shaped trim boards or portions thereof wherein no holes have to be cut.
Another solution for reducing the formation of cracks when press-forming corrugated paperboard as substrate for automobile roof trim or door trim boards is disclosed in GB 1 554 077. This solution consists in that the corrugated paperboard is moistened by means of water or steam prior to the press-forming step. A similar method is disclosed in U.S. Pat. No. 6,596,124 for a cellulose based paper honeycomb structure. In this method the honeycomb structure is immersed in water for several minutes to a few hours until it is flexible enough to be moulded into the desired shape. A drawback of this method is that, especially for complex shapes, a relatively long drying period is required to dry the honeycomb structure before the covering layers can be applied and secondly, that for complex shapes vacuum needs to be applied during moulding. A further drawback is that a mould made of a metal mesh material is needed in order to allow hot air to pass through the cells to dry the paper honeycomb material. Covering layers can thus not be moulded directly onto the honeycomb structure in the compression mould, but need to be applied afterwards. The presence of such covering layers would indeed prevent or hamper at least the drying of the core material and it is not possible to mould the cover layers against a mould surface made of an open mesh material. Moistening methods as described in GB 1 554 077 and U.S. Pat. No. 6,596,124 generally improve the ductility of the core material so that it can be shaped to some extent, more particularly to two-dimensionally curved planes, but they are insufficient to improve the ductility of the core material to such an extent that it can be moulded into complex, three dimensional shapes without cracking or rupture problems or loss of mechanical properties. Furthermore, according to U.S. Pat. No. 6,596,124, paper-based core materials need to be pretreated with a resin coating prior to the forming process in order to allow the core material to take up the required amount of moisture without reducing the mechanical properties of the core material too much.
Other methods for reducing cracks when moulding or shaping honeycomb core materials are disclosed in U.S. Pat. No. 5,824,255 and in DE-A-42 33 139. In these methods the sheets of honeycomb material are only curved two-dimensionally. During the curving step, the honeycomb material is supported by a carrier sheet or plate which is curved simultaneously with the honeycomb material to reduce the formation of cracks therein. A drawback of these methods is that they are only applicable for relatively simple shapes. The required three-dimensionally shaped structures are often required to meet the demand of a high design freedom for the production of for instance vehicle interior parts, such as headliners, instrument panels, door panels, etc. Such vehicle interior parts can be produced for instance by spraying a skin on a mould and uniting this skin to a structural carrier or substrate layer by means of an intermediate foam layer. Reference is for instance made to WO 02/09977; WO 02/26461 and WO 05/021230. Substrate layers are mostly made of plastics or reinforced plastics such as PU, PP, ABS, PE, ABS/PC and PC. As there is a need for more lightweight substrate materials, one has to take into account the mouldabilty of these materials, in order to make them usable as for instance substrate materials for vehicle trim parts.